Gear differentials generally include compound planetary gear sets interconnecting a pair of drive axles to permit the latter to rotate in opposite directions with respect to a differential housing. The drive axles rotate about a common axis; and so-called "side" gears are fixed for rotation with the inner ends of the two drive axles, such side gears acting as the sun gear members of the compound planetary gear sets. The sun gears are interconnected by so-called "element" or "combination" gears, which form the planet gear members of the sets. The planet gears are mounted for rotation about axes that may be variously offset and inclined with respect to a common axis of the sun gears and drive shafts.
The relative positions of the sun and planet gear axes usually determine the kind of gearing that make up the planetary gear sets: Parallel axes are used for mounting spur or helical gears, e.g., see U.S. Pat. No. 2,269,734 (Powell), U.S. Pat. No. 2,382,846 (Barber), and U.S. Pat. No. 3,768,336 (Wharton); and orthogonal axes are used for mounting either bevel or worm gears, depending upon the presence of any offset between the axes. That is, bevel gears are used when the sun and planet gear axes intersect, while worm gears are used when the gear axes do not intersect (as an example of this latter type, see U.S. Pat. No. 1,373,657 to Finefrock).
The entire planetary gearing arrangement within the differential supports opposite relative rotation between the drive axle ends (i.e., differentiation), which is necessary to permit the axle ends to be driven at different speeds. Torque transmitted to the drive axles through the inclined tooth surfaces of the sun/side gears generates thrust forces against gear-mounting bearing surfaces within the differential. (Such bearing surfaces may comprise journals formed in the housing, or may be the ends of bores into which the gears are received, or may be special washers positioned between the end faces or shaft ends of the gears and the housing.) The thrust forces, together with other loads conveyed by the gear meshes in the planetary gearing, produce a frictional resistance to relative rotation between the drive axles, this frictional resistance being proportional to the torque applied to the differential housing. The proportional frictional resistance supports different amounts of torque between the two drive axles to prevent their relative rotation until the characteristic "bias" ratio of the planetary gearing arrangement is reached. Once the frictional resistance is overcome and differentiation begins, the torque difference between the axles is proportioned in accordance with the bias ratio. Differentials that divide torque in a substantially constant ratio between relatively rotating drive axles, are referred to as "torque-proportioning" differentials.
The ability to support different amounts of torque between the drive axles is of great benefit to improving traction capabilities of vehicles. Ordinarily, when one wheel of a vehicle with a conventional differential loses traction, the amount of torque that can be delivered to the other drive wheel is similarly reduced. However, when one wheel loses traction so that there is differentiation between the two axles, torque-proportioning differentials deliver an increased amount of torque to the drive wheel having better traction, such increased torque being determined by the characteristic bias ratio of the differential.
Some well-known torque-proportioning differentials use planetary gearing assemblies with orthogonal axes, while others use gearing assemblies with parallel axes. Examples of the latter type are U.S. Pat. No. 1,938,649 (Welsh), U.S. Pat. No. 2,000,223 (DuPras), and U.S. Pat. No. 2,479,638 (Randall). The above-cited Finefrock patent is an example of a torque-proportioning differential using orthogonal axes and worm gearing.
In general, the gears used in parallel-axis/helical-gear assemblies are usually simpler to manufacture than are the gears used in torque-proportioning designs of the orthogonal-axis/worm-gear type. However, when the latter are made with the same number of sun and planet elements as the former, they usually develop greater frictional resistance between their respective gear meshes and support bearings; and this, in turn, provides greater torque bias and/or increased control over the bias ratio. In differentials of the worm-gear type, the sun/side gears are a pair of helical worm gears which are interconnected by a plurality of so-called element gears that are mounted in planetary pairs within the body portion of the housing. The axes of rotation of each element gear of a pair are parallel to each other, but are crossed and nonintersecting with respect to the common axis of rotation of the side gears. Typically, the side gears are interconnected by three such pairs of element gears mounted in the housing at even angular increments about the periphery of the side gears.
The planet/element gears of the worm-gear type differential are in reality combination gears, i.e., the middle portion of each element gear is formed as a worm wheel, while its respective axial end portions are formed as spur gears. The gear arrangement of the worm-gear type torque-proportioning differential is such that: for any given pair of element gears, the worm wheel portion of a first element gear meshes with one side gear; the worm wheel portion of a second element gear of the pair meshes with the other side gear; and the spur gear portions of the respective element gears mesh with each other.
The typical parallel-type torque-proportioning differential (such as that disclosed in the DuPras reference cited above) also fixes sun/side gears to the ends of the two drive axles and interconnects these side gears with pairs of planet gears. Each planet gear is in mesh with its paired planet gear; and each gear in the pair meshes, respectively, with one of the side gears. However, in contrast with the worm-gear type element gears just described above, in parallel-axis assemblies the planet gears are not formed with a middle portion and two axial end portions. Instead, one axial end of each individual planet gear is in mesh with its respective side gear, and its other axial end is in mesh with its paired planet gear. This form of planetary gear is also used in those parallel-axis differentials which, instead of pairs of planetary gears, mount the planetary gears in a continuous circular mesh around the full circumference of each respective side gear, e.g., see U.S. Pat. No. 1,869,528 (Trbojevich) and U.S. Pat. No. 3,738,192 (Belansky). That is, in all known parallel-axis torque-proportioning differentials, the planetary gear pairs mesh with each other at only one of their axial ends, and their respective loads are often carried primarily by only one end of their axial mounting supports.
My invention improves the load balance on the planetary gear mounting supports of such parallel-axis torque-proportioning differentials and, additionally, provides such differentials with an increase in frictional surfaces, thereby providing greater control over bias ratio. Further, my parallel-axis differential includes novel planetary gear pairs having separated double-meshing engagement such as that which has only been known heretofore in worm-type, orthogonal-axis differentials.